Minimal mass, less gas

December 2012 - The holiday season brings loved ones together for fattening sweets, gift exchanges and all things pleasant. It’s also that time of year when consumers are bombarded with unrelenting car commercials bragging about fuel economy and drivers who forget how to pump gas. However, these advertisements reflect a serious American demand for vehicles with better fuel efficiency.

Developing lightweight material for car manufacturing can significantly improve fuel efficiency. The Materials Genome Initiative, a part of the White House’s Reducing Oil Dependence for Transportation Initiative, strives to find weight solutions for automobiles. According to the initiative, “every 10 percent reduction in passenger vehicle weight in a conventional combustion engine car could reduce fuel use by 6 to 8 percent.”

To decrease vehicle mass, General Motors is testing a magnesium sheet metal thermal-forming process as an alternative to aluminum and steel. According to the company, magnesium weighs 33 percent less than aluminum, 60 percent less than titanium and 75 percent less than steel.

A new use for magnesium

However, magnesium was unreliable in the past. Automakers used traditional panel forming methods and therefore struggled to make the sheet metal strong and non-corrosive. “Magnesium sheet is essentially very difficult to form at room temperature,” says Jon T. Carter, GM R&D metals researcher. “Also, in many applications it requires some kind of good coating on it for protection against corrosion while it’s in service.”

Automotive applications have used magnesium die castings for years, but GM specifically is researching rolled magnesium sheet metal, explains Carter, “where you get the shape by a forming operation rather than a casting operation.”

GM’s process heats magnesium to 842 degrees Fahrenheit, molding the material into precise, rigid shapes. This process, combined with a corrosion-resistant surface preparation, makes the magnesium sheet stronger. “The edge preparation particularly gets done before the coatings go on,” says Carter. “It’s the edge preparation that we do that gives us uniform coating thickness and therefore allows that coating to be protective.”

Magnesium is used in smaller car parts like steering wheels and engine cradles, but GM recently developed a production-ready rear deck lid inner panel using the sheet metal. By replacing a steel deck lid inner panel with magnesium, GM can remove 2.2 pounds of weight, while remaining strong enough to withstand 77,000 robotic slams and 550-pound impact drops.

A magnesium process

The process for magnesium, “involves only partially heating the blank before it goes into a forming press and then doing the final bit of heating in the forming press while the forming is underway,” says Carter. “By using that kind of process, rather than having the blank fully preheated before it goes into the forming press, we’re able to form the parts faster and get a higher yield out of the press.”

GM developed a process for magnesium by adapting its existing aluminum thermal-forming process through laboratory experiments and plant trials. The preheat temperature for magnesium is lower than aluminum, while the forming time is slightly longer. “The aluminum sheet that’s typically used for this type of process is specially developed for that process,” says Carter. “The magnesium sheet that we use was actually commercially available off the shelf and has not yet been optimized for that forming process. So I would think with some optimization of that magnesium sheet, we could form pieces even faster.”

Carter also noticed a change in the magnesium sheet producing industry that could make the metal a cost-competitive product. “They were starting to adopt the continuous-casting process instead of direct-chill casting process, which was forecast to bring the price of magnesium sheet down significantly,” he says. “We wanted to have our process developed for forming the sheet by the time the new sheet production process [became available] so we’d be able to utilize that lower price sheet.”

Massive savings

GM plans to expand its magnesium offering by experimenting with magnesium car doors, hoods and possibly C-frames. According to the company, the U.S. Automotive Materials partnership estimates by 2020, “350 pounds of magnesium will replace 500 pounds of steel and 130 pounds of aluminum per vehicle, an overall weight reduction of 15 percent. This weight savings would lead to a fuel savings of 9 to 12 percent.”

“I think certainly we would stand to get a major weight savings, which is really our goal in using magnesium and other lightweight materials,” says Carter, noting that magnesium could be cost-competitive to “carbon-fiber reinforced plastic materials, which is also a leading candidate for weight reduction.” He also notes magnesium is another tool for GM’s designers to work with.

The trick to lightweighting a vehicle is to maintain its strength. “Each part of the car has to serve a purpose,” says Carter, “passenger protection being number one, [the car] has to do that job regardless of what it’s made of.”MM